Previous Projects

Solar Cells: Simulation of tunnel junctions (Alexandre Walker)Modeling and simulation of III-V semiconductor multi-junction solar cells is invaluable to achieve higher efficiency devices. To realize this, a fully functional three-dimensional model of a lattice-matched multi-junction solar cell is required. The first steps include decomposing the various components of such a complex device and accurately simulating each individually. Once each component is understood and optimized as an individual device, the entire structure can be simulated and optimized altogether. This research develops such a model using the semiconductor simulation software package Synopsys Sentaurus. Current progress focuses on the accurate simulation of tunnel junctions, which are the optically-transparent, electrically-conductive components coupling adjacent sub-cells. The figure below shows the simulated characterization of an AlGaAs tunnel junction, in strong agreement with an experimentally characterized tunnel junction. The negative differential resistance (NDR) region is an unstable region due to parasitic capacitance and inductance effects of the tunnel junction which leads to instability during experimental measurement. Future research will focus on simulating individual solar cells and moving to dual and triple junction solar cells with appropriate tunnel junction designs. The next aspects of this research will incorporate the quantum dots within the middle sub-cell to achieve improved current balance across all sub-cells.

Solar Cells: Tunnel Junctions current characteristics (Gitanjali Kolhatkar)One of the major challenges in multi-junction solar cell design is to conduct current between each p-n junction. This can be accomplished by introducing tunnel junctions, which interconnect the three p-n junctions. A tunnel junction is a highly doped n-p junction that connects the n-terminal of one sub-cell to the p-terminal of the adjacent sub-cell. These tunnel junctions are still not well understood and require testing to determine the most appropriate design to achieve the highest overall efficiency in the multi-junction solar cell. This project consists of experimentally measuring current-voltage characteristics of tunnel junctions (ex. AlGaAs/AlGaAs) under different conditions, including the variation of temperature and their time-dependent behavior. The results of this work help to calibrate numerical models developed within our research group. Triple junction GaInP/InGaAs/Ge solar cells which incorporate these tunnel junctions are also tested under low concentration artificial sunlight using an Oriel solar simulator capable of achieving intensities of up to 150 suns.

Solar Cells: Receiver Thermal Management (Simon Chow)Solar cells under continuous concentrated illumination produce a great amount of heat within the system. Commercially-available concentrator systems are designed to illuminate solar cells with the intensity of ~500 suns onto a single solar cell, with the near-future goal to move to >1000 suns to reduce the photovoltaic device costs. However, high operating temperatures – which are induced under high illumination intensities – generally reduce the conversion efficiency of the photovoltaic cell. The solar cell receiver provides the mean to transport both heat and electrical power produced by the solar cell. The goal of this research is optimize the design of a solar cell chip-on-carrier receiver through both experiments and analytical simulations.

LC-DFB: Laterally-Coupled Distributed Feedback Lasers (Ron Millett)Laterally-coupled (or corrugated ridge) distributed feedback lasers use a grating that is patterned directly out of the waveguide ridge. They can be fabricated without requiring additional regrowth steps, and easily monolithically integrated with a variety of other photonic devices. The strict grating requirements of LC-DFB lasers can be relaxed by using higher order gratings that use longer grating periods compared to first-order gratings. A comprehensive analysis of LC-DFB laser properties has suggested grating geometries that offer both high-performance and good fabrication-tolerance. Dual-wavelength LC-DFB laser designs, with different grating periods patterned on either side of the waveguide ridge, have shown potential as sources for microwave signals.